The invention relates to a drive-capable support or traction means and a method for production thereof.
Cables of steel are known as support or traction means, where several strands of steel are laid to form a cable. In the following, support or traction means are discussed as drive-capable cables or belts for the realm of elevators. With knowledge of the invention, applications of drive-capable support or traction means are entirely possible in other technical fields, such as, for example, in cranes. In the field of elevators, support or traction means are driven by friction contact at drive pulleys or are deflected at deflecting rollers, wherein the strands are loaded in bending and tension and absorb traction forces. Cages or counterweights connected with these support or traction means are driven in this manner in elevator shafts.
The drive capability of support means is fixed in European Standard EN-81-1 in paragraph 9.3, wherein the minimum requirements read as follows: a) the cage must be held, without slipping, at a stopping point when it is loaded with 125% of the nominal load, b) in the case of emergency braking the empty cage, or cage loaded with nominal load, must be decelerated at a speed which does not lie above the design speed of the buffer, inclusive of a reduced buffer stroke, and c) the empty cage should not rise when the counterweight rests on buffers and the drive runs in an upward direction.
A cable of synthetic fiber material with the objects of reducing the weight of such support or traction means and in order to provide a largely maintenance-free support or traction means has become known from EP-0672781. In this cable, strands of aramid are laid in two layers with a lefthand start and righthand start and completely encased by an outer casing of polyurethane (PU). The outer casing is fixedly connected with the outer strand layer and has such a high bonding force relative to the outer strand layer that an introduction of the traction forces from the drive pulley through the outer casing into the outer strand layer is made possible without the outer strand layer in that case displacing or compressing. The requirements with respect to service life of drive-capable support or traction means are high, under which they shall survive at least 105, if not 106, bending loads. Disposed between the outer strand layers and an inner strand layer there is a friction-reducing intermediate casing which ensures a relative movement of the outer strand layer with respect to the inner strand layer during bending at the drive pulley and, through this relative movement, avoids wear, which is disadvantageous for the strands, by mutual rubbing. The intermediate casing separates the outer strand layer from the inner strand layer.
It has now proved disadvantageous in the first instance that in the case of such a drive-capable cable of synthetic fiber material the size of the coefficient of friction of the strands relative to one another is not settable in a controlled manner. EP-0672781 does indeed disclose the use of silicon as lubricant of the strands relative to one another, but diffused silicon readily escapes through the outer casing of PU or from the cable, which in turn, as soon as silicon gets on the drive pulley, limits the drive capability of the cable and thus the usability of the elevator.
Moreover, it has proved disadvantageous that an over-dimensioning of the coefficient of friction leads to development of heat or to prematurely occurring stiction, while an under-dimensioning of the coefficient of friction leads to twisting of the strands in the outer casing (cable twisting). These effects of heat development, premature stiction and cable twisting reduce the service life of the cable of synthetic fibre material.
As a solution for these disadvantages there is disclosed in U.S. Pat. No. 5,881,843 a cable for elevators in which the strands of aramid are laid in several strand layers to form a cable and are encased by an outer casing of polyamide (PA) or PU. Each of the strands is additionally protected by a strand casing of PA or PU. The coefficient of friction between the outer casing and the strand casings is greater than or equal to 0.15 and thus allows introduction of traction forces from the drive pulley through the outer casing into the strand casing of the strands of the outer strand layer, while the coefficient of friction between the strand casings is greater than or equal to 0.10 and thus facilitates movement of the strands relative to one another.
The low bonding force between the outer casing and the outer strand layer is a disadvantage of the teaching according to U.S. Pat. No. 5,881,843, whereby introduction of traction forces from the drive pulley through the outer casing into the outer strand layer is not possible without displacement or compression of the outer strand layer. The expensive production process of the cable of synthetic fiber material, where each strand is provided with a strand casing, is also disadvantageous. On the other hand, the increase in the cable diameter by the strand casing is disadvantageous, this strand casing producing a mutual spacing of the strands so that the strands cannot rub directly against one another.